Underground drilling involves drilling a bore through a formation deep in the Earth using a drill bit connected to a drill string. During rotary drilling, the torque applied at a top drive of a drilling rig is often out of phase with the rotational movement at the bottom-hole assembly (BHA) of the drill string due to an elasticity of the material of the drill string. This causes the drill string to yield somewhat under the opposing loads imposed by the rotational force at the top drive and friction/inertia at the end where the bit is located (e.g., the BHA). This causes resonant motion to occur between the top drive and the BHA that is undesirable. Further, as the drill string winds up along its length due to the ends being out of phase, the torque stored in the winding may exceed any static friction, causing the drill string near the bit to slip relative to the wellbore sides at a high (and often damaging) speed.
Existing approaches to mitigating stick-slip modulate the rotations per minute (RPM) of a top drive of the drilling rig in order to mitigate vibrations occurring down hole, with the goal of keeping a constant, smooth torque at the top drive quill as much as possible. Therefore, these existing approaches modulate RPM to achieve a smooth torque response. To accomplish this, controllers that manage stick-slip mitigation typically utilize a speed control loop in the controller, e.g. an alternating current (AC) drive. However, speed control loops are slower than torque or current control loops in AC drives. The resulting delay of speed control loops in generating RPM commands, and therefrom new torque commands, affects the performance of the stick-slip mitigation system at higher frequencies. This limits the ability of existing approaches to mitigate stick-slip at higher harmonics.
The present disclosure is directed to systems, devices, and methods that overcome one or more of the shortcomings of the prior art.